I. Field of the Invention
The present invention relates generally to methods and apparatus for high recovery of hydrocarbon liquids from methane-rich natural gases and other gases, e.g., refinery gases. More particularly, the present invention provides methods and apparatus for more efficiently and economically achieving high recovery of ethane, propane, propylene and heavier hydrocarbon liquids (C2+ hydrocarbons) in association with liquified natural gas production.
II. Description of the Background
Due to its clean burning characteristics and the implementation of more stringent environmental regulations, the projected demand for natural gas has been increasing during recent years. In addition to methane, natural gas includes some heavier hydrocarbons and impurities, e.g., carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. After compression and separation of these impurities, natural gas may be further processed to separate and recover heavier hydrocarbons as natural gas liquids (NGL) and produce pipeline quality methane. The pipeline quality methane is then delivered to gas pipelines as the sales gas ultimately transmitted to end-users.
In the case of remote gas production or distant gas markets, transportation of produced natural gas via gas pipeline might not be economical or even feasible. Accordingly, liquifaction of natural gas has become a viable and widely adopted scheme. The economics of liquifying natural gas is feasible due mainly to the great reduction in volume as the gas is converted to a liquified state, making it easy to store and transport. Another advantage of converting the produced natural gas to a liquified form is that the produced LNG can be economically stored to supplement energy suppliers during seasonal peak demand periods. Liquified natural gas, typically stored at atmospheric pressures and at temperatures of approximately xe2x88x92260xc2x0 F., is transported to distant markets via refrigerated tankers.
Processes for the liquifaction of natural gas are well known in the art. Natural gas comprising predominantly methane enters an LNG plant at elevated pressures and is pretreated to produce a purified feed stock suitable for liquifaction at cryogenic temperatures. The pretreatment typically includes removal of acid gases, e.g., hydrogen sulfide and carbon dioxide, together with other contaminants, including moisture and mercury. The purified gas is thereafter processed through a plurality of cooling stages using indirect heat exchange with one or more refrigerants to progressively reduce its temperature until total liquifaction is achieved. The pressurized liquid natural gas is sub-cooled to reduce flashed vapor through one or more expansion stages to final atmospheric pressure suitable for storage and transportation. The flashed vapor from each expansion stage, together with the boil off gas produced as a result of heat gain, are collected and used as a source of plant fuel gas with any excess recycled to the liquifaction process.
Because a significant amount of refrigeration energy is required for liquifying natural gas, the refrigeration system becomes one of the major units in an LNG facility. Mechanical refrigeration cycles mostly in closed circuit are often employed in LNG projects. A number of liquifaction processes have been developed with the differences mainly found in the refrigeration cycles used. The most commonly used LNG processes can be classified into three categories as follows:
1) The cascade process presenting the benefits of easy start-up or shutdown. The cascade process consists of successive refrigeration cycles using propane, ethane or ethylene, and methane. The thermal efficiency can be readily enhanced by the use of multi-compressor stages. U.S. Pat. No. 5,669,234, incorporated herein by reference, represents an exemplary cascade process.
2) The propane pre-cooled mixed refrigerant process involves the use of a multi-component mixture of hydrocarbons, typically comprising propane, ethane, methane, and optionally other light components in one cycle, and a separate propane refrigeration cycle to provide pre-cooling of natural gas and the mixed refrigerant to approximately xe2x88x9235xc2x0 F. The propane mixed refrigerant process advantageously provides improved thermal efficiency. However, a significant disadvantage results from the use of extremely large spiral wound exchangers. Such exchangers are a long lead item requiring special facilities in the field to manufacture. Examples of the propane mixed refrigerant process include those disclosed in U.S. Pat. Nos. 4,404,008 and 4,445,916, incorporated herein by reference.
3) The single, mixed refrigerant process includes heavier hydrocarbons, e.g., butanes and pentanes, in the multi-component mixture and eliminates the pre-cooled propane refrigeration cycle. It presents the simplicity of single compression in the heat exchanger line and is particularly advantageous for small LNG plants. U.S. Pat. No. 4,033,735, incorporated herein by reference, represents an exemplary single, mixed refrigerant process.
The use of a turbo expander in combination with mechanical refrigeration cycles has also been adopted in many LNG processes. Examples of the use of a turbo expander are disclosed in U.S. Pat. Nos. 3,724,226; 4,065,278; 5,755,114; 4,970,867, 5,537,827; and Int""l Patent No. WO 95/27179.
In addition to methane, natural gas typically contains various amounts of ethane, propane and heavier hydrocarbons. The composition varies significantly depending on the source of the gas and gas reserve characteristics. Hydrocarbons heavier than methane need to be removed from LNG for various reasons. Hydrocarbons heavier than pentane, including aromatics, having high freezing points must be reduced to an extremely low level to prevent the freezing and plugging of process equipment in the course of cooling and liquifaction steps. After separation of these heavy components from LNG, they provide excellent gasoline blending stock. Many patents have been directed to methods for removal of these heavy hydrocarbons. For instance, U.S. Pat. No. 5,325,673 discloses the use of a single scrub column in the pretreatment step operated substantially as an absorption column to remove freezable C5+ components from a natural gas stream feeding to an LNG facility. The heavy liquid recovered subsequently can be fractionated into various fractions for use as make-up refrigerants. U.S. Pat. No. 5,737,940 describes an exemplary system incorporated in a cascade process.
Besides being liquified as part of LNG and used as fuel, lighter natural gas liquid (NGL) components, e.g., hydrocarbons having 2-4 carbon atoms, can also be a source of feedstock to refineries or petrochemical plants. Therefore, it is often desirable to maximize the recovery of NGL to enhance revenue. To achieve high recovery of these components, it is common practice to design an NGL recovery plant so that the tail gas produced by the NGL recovery plant and comprising primarily methane is delivered to the LNG facility for liquifaction. U.S. Pat. Nos. 5,291,736 and 5,950,453 are typical examples of such combined facilities.
Among several different NGL recovery processes, the cryogenic expansion process has become the preferred process for deep hydrocarbon liquid recovery. In a conventional turbo-expander process, the feed gas at elevated pressure is pretreated for the removal of acid gases, moisture and other contaminants to produce a purified feed stock suitable for further processing at cryogenic temperatures. The purified feed gas is then cooled to partial condensation by heat exchange with other process streams and/or external propane refrigeration, depending upon the richness of the gas. The condensed liquid after removal of the less volatile components is then separated and fed to a fractionation column, operated at medium or low pressure, to recover the heavy hydrocarbon constituents desired. The remaining non-condensed vapor portion is turbo-expanded to a lower pressure, resulting in further cooling and additional liquid condensation. With the expander discharge pressure typically the same as the column pressure, the resultant two-phase stream is fed to the top section of the fractionation column where the cold liquids act as the top reflux to enhance recovery of heavier hydrocarbon components. The remaining vapor combines with the column overhead as a residue gas, which is then recompressed to a higher pressure suitable for pipeline delivery or for liquifaction in an LNG facility after being heated to recover available refrigeration.
Because a column operated as described above acts mainly as a stripping column, the expander discharge vapor leaving the column overhead that is not subject to rectification still contains many heavy components. These components could be further recovered through an additional rectification step. Ongoing efforts attempting to achieve a higher liquid recovery have mostly concentrated on the addition of a rectification section and the generation of an enhanced reflux stream to the expanded vapor. Many patents exist purporting to disclose an improved design for recovering ethane and heavier components in an NGL plant. For example, see U.S. Pat. Nos. 4,140,504; 4,251,249; 4,278,457; 4,657,571; 4,690,702; 4,687,499; 4,851,020; and 5,568,737. At best, these processes are capable of recovering 95%+ of ethane and heavier hydrocarbons. However, they typically involve a significant capital expenditure during construction of the NGL plant as well as increased operational costs during its lifetime.
It will be recognized that all NGL components have higher condensing temperatures than methane so that all will be liquified in the course of operating an LNG process. A substantial cost savings may be realized, if the NGL recovery could be effectively integrated within the liquifaction process instead of building a separate facility.
Recovery of NGL in the LNG facility has also been suggested in the literature. For example, it has been suggested that lighter NGL components could be recovered in conjunction with the removal of C5+ hydrocarbons by using a scrub column in a propane pre-cooled, mixed refrigerant process. See U.S. Pat. Nos. 4,445,917 and 5,325,673. A cryogenic stripping column in a cascaded process was suggested in U.S. Pat. No. 5,737,940 for recovery of heavy hydrocarbons from a natural gas feed stream. In a further modification, U.S. Pat. Nos. 5,950,453 and 5,016,665 disclose a method wherein a demethanizer is incorporated in the process for liquifying natural gas for recovering heavier hydrocarbon liquids.
The NGL recovery column in these systems is often required to operate at a relatively high pressure, typically above 550 psig, in order to maintain an efficient and economical utilization of mechanical refrigeration employed in the LNG process. While benefitting from lower refrigeration energy by maintaining a high liquifaction pressure, the separation efficiency within the recovery column may be significantly reduced due to less favorable separating conditions, i.e., lower relative volatility inside the column. In addition, prior art processes fail to effectively provide reflux to the recovery column. As a result, none of these processes are capable of efficiently maintaining a high NGL recovery, i.e., the NGL recovery does not typically exceed 80% with these processes.
As can be seen from the foregoing description, those skilled in the art have long sought methods and apparatus for improving the efficiency and economy of processes for separating and recovering ethane and heavier natural gas liquids in an NGL plant. While prior art methods have been capable of recovering more than 95% of the ethane and heavier hydrocarbons in a stand-alone NGL recovery plant, those processes fail to maintain the same recovery when integrated with an LNG facility. Accordingly, there has been a long felt but unfulfilled need for more efficient, more economical methods of integrating these processes while improving, or at least maintaining, their economics.
The present invention provides an integrated process for recovery of the components of a feed gas containing methane and heavier hydrocarbons while maximizing NGL recovery and minimizing capital expenditures and operating costs incurred with the LNG facility. The present invention is also intended to improve separation efficiency within an NGL recovery column while maintaining column pressure as high as practically possible to achieve an efficient and economical utilization of mechanical refrigeration in the liquifaction process. This is achieved by the introduction of an enhanced liquid reflux specifically suitable for the purpose of the recovery column.
Historically, the price of liquid ethane has been cyclical, rising and falling in response to the demand for use as petrochemical feed stock. When the price of liquid ethane is high, gas processors can generate additional revenues by increasing the recovery of ethane. On the other hand, when the ethane market is depressed, it may be desirable to effectively reject ethane, allowing it to remain in the LNG, but still maintain high recovery of propane and heavier components. Due to the cyclical nature of the liquid ethane market, designing a facility which can selectively and efficiently recover or reject ethane will allow producers to quickly respond to changing market conditions, a phenomenon that seems to occur ever more frequently in today""s market. Accordingly, the present invention is designed to permit flexible transition between operation for ethane recovery or ethane rejection.
A number of liquifaction processes developed in the prior art have been described above. These processes may differ significantly depending on the mechanical refrigeration cycle used. The methods of the present invention may be integrated with any of those processes. The methods of the present invention are applicable independent of the type of mechanical refrigeration used in the LNG process.
The present invention, in the broadest sense, provides an integrated process and apparatus for cryogenically recovering ethane, propane and heavier components during natural gas liquifaction processes via a distillation column, in which the reflux derived from various sources in the liquifaction process is essentially free of the components to be recovered. The provision of an enhanced liquid reflux, which is lean on the NGL components, to the distillation column permits a high recovery of NGL components even when the column is operated at a relatively high pressure. The process involves introducing a cooled gas/condensate feed into a first distillation column, e.g., an NGL recovery column, at one or more feed trays. The gas/condensate feed is separated into a first liquid stream primarily comprising NGL components to be recovered and a methane rich overhead stream essentially free of NGL components. The methane-rich overhead stream is further cooled to total liquifaction. Preferably the liquified methane-rich stream is further sub-cooled. This liquified, and preferably sub-cooled, methane-rich stream under pressure is subsequently flashed to near atmospheric pressure in one or more steps with the liquid collected in the final flash step being delivered to the LNG tank for storage. The flashed vapor is heated and compressed to a higher pressure for delivery as fuel gas. Excess flashed vapor, if any, is recycled to the liquifaction process in which it is ultimately liquified as pressurized LNG or as liquid reflux to the NGL recovery column. The first liquid stream is introduced into a second distillation column, e.g., an NGL purifying column, at one or more feed trays. In the second column, the first liquid stream is separated into an NGL product stream from the bottom and a first vapor portion primarily comprising all of the remaining lighter components from the overhead.
In one embodiment of the present invention, the first vapor portion is combined with at least a portion of the excess flashed vapor. The combined stream is compressed and cooled to substantial condensation and thereafter introduced to the top of the NGL recovery column as a liquid reflux. This reflux stream will contain an extremely low concentration of the heavy components to be recovered in the NGL product. This stream enhances the recovery efficiency within the column and reduces the loss of NGL components in the methane-rich overhead stream to a minimum. A high NGL recovery is therefore achieved even with a relatively high operating pressure, i.e., a pressure of about 600 psig, for the NGL recovery column.
The economic advantages of the present invention can be further improved by thermally linking a side reboiler for the first distillation column with the overhead condenser for the second distillation column. More specifically, the first vapor portion is cooled in countercurrent heat exchange with a liquid withdrawn from a tray located below the feed trays of the first distillation column. The cooled first vapor portion is separated into a liquid fraction for introduction into the second distillation column as a top liquid reflux and a lighter, vapor fraction with further reduced NGL components for introduction into the first distillation column as a top reflux. Thus the NGL recovery efficiency in the second column is enhanced. The heat carried by the liquid withdrawn returns to the first distillation column where it provides a stripping action in the bottom portion of the column, thereby reducing volatile components, e.g., methane, in the first liquid stream from the bottom.
The recovery efficiency can be improved in another embodiment of the present invention by introduction of a second liquid reflux to the upper, rectification section of the first distillation column. The second reflux enters the distillation column preferably in the middle of the rectification section, as a middle reflux which provides a bulk rectification effect and reduces the NGL components to be recovered in the up-flow vapor stream. Any residual NGL components in the upward vapor stream can be recovered by the top and leaner liquid reflux. A slipstream from the feed gas can be taken and cooled to substantial condensation or even sub-cooling to form the second liquid reflux. In some cases, the feed gas contains much heavier components, e.g., hexane and aromatics, which tend to freeze at cryogenic temperatures. The feed gas can be first cooled to partial condensation where most of these components will be condensed in the liquid and separated out in a separator. A slipstream can then be taken from the non-condensed vapor portion and further cooled to substantial condensation to form the second liquid reflux. Optionally, this liquid reflux can be fed to the top of the NGL recovery column.
In another embodiment of the present invention, the top reflux to the first distillation column is generated by recycling a small portion of LNG under pressure prior to undergoing flashing. This reflux also has an extremely low content of the NGL components to be recovered and, accordingly, enhances separation efficiency. This reflux scheme can be advantageous for the liquifaction process where the LNG can be deeply sub-cooled using very cold mechanical refrigeration to reduce the vapor produced in the flashing steps to a minimum. Typical examples of this embodiment include liquifaction processes using mixed refrigerant with or without propane pre-cooling, cascaded refrigeration in a closed circuit.
Another feature providing a significant economic advantage in the present invention is the cooling of the feed gas by countercurrent heat exchange with a refrigerant stream comprising a portion of the first liquid stream or liquid withdrawn from the lower portion of the first distillation column. As a result, the refrigerant stream is partially vaporized and may be separated into a second liquid stream for introduction into the second distillation column and a second vapor stream for introduction into the first distillation column as a stripping gas after compression and cooling. The introduction of stripping gas supplements the heat requirements in the first distillation column for stripping volatile components off the first liquid stream. It also enhances the relative volatility of the key components and, accordingly, the separation efficiency in the column, particularly when the column is operated at a relatively high pressure as in the NGL recovery column of the present invention.
The methods and apparatus of the present invention efficiently integrate NGL recovery into the natural gas liquifaction process and permit high recoveries of propane and heavier components, e.g., recoveries exceeding 95% of those components originally present in the feed gas. In fact, the methods of the present invention, properly optimized, permit the recovery of at least 99% of the propane and heavier hydrocarbons originally found in the feed gas. The high recovery of heavier hydrocarbons achieved with the methods of the present invention may be advantageously used to clean gas feeds contaminated by cyclohexane, benzene and other heavy hydrocarbons which have been determined to create potential freezing problems and, accordingly, must be thoroughly removed. This high NGL recovery is achieved while eliminating the NGL plant, as typically employed in the prior art, in the front-end of the LNG facility. Thus, significant savings of capital, as well as operating costs, are achieved. In addition, the flexible design of the present invention permits an easy transition between operations designed to either recover or reject ethane in order to accommodate rapidly changing values of liquid ethane. The integration methods proposed in the present invention can also be easily adapted for use with any liquifaction process regardless the refrigeration system used.